tests: make linux v5.1 the minimum version to run tests
tests: make linux v5.1 the minimum version to run tests

The testing framework has been adjusted for linux v5.1. It will no
longer work with older versions.

Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod